Chemical reactors typically define a large volume within which reactants are placed. The only surfaces these reactants encounter are the sides of the reaction vessel, therefore imposing heat and mass transport limitations. Slow reaction rates result.
Microchannel reactors reduce the size of conventional chemical reactors without lowering the throughput. The distance between heat generation and removal is reduced from tens of centimeters in conventional reactors to tens of microns (μ) in microchannel reactors.
Microchannel reactors are fluidic devices that rely on embedded microstructures, of much smaller length scales (typically 500-1000 micrometers (μm)) than traditional systems, for their function. For thermal and chemical applications, small architecture and geometry provide the benefits of high rates of heat and mass transfer, large surface-to-volume ratios, and the opportunity of operating at elevated pressures. For other applications, small dimensions imply rapid response and compact design. The compact design is of value in space/weight sensitive applications such as transportation.
Further, unlike conventional chemical reactors, microchannel reactors number-up to process (not scale-up). In order to process larger volume of materials, channels are added to the reactor by either making the device larger, or by arranging multiple reactors in parallel. Potential applications of micro-reactors include, but are not limited to, heat exchangers, recuperators, heat-pumps, chemical separators, chemical reactors, fuel processing units, and combustors.
Microchannel reactors can be manufactured by the diffusion bonding of precision machined metallic foils or sheets. This method of manufacture is described in U.S. Pat. Nos. 5,811,062, awarded on Sep. 22, 1998; and 5,611,214, awarded on Jul. 29, 1997, respectively, both to R. S. Wegent et al.
The manufacture of microchannel reactors via diffusion bonding consists of three basic steps: forming, registration, and bonding. Forming consists of precision machining the internal features of the reactor in each foil. Machining can be accomplished through laser ablation, chemical machining, or mechanical methods such as punching or machining.
Registration is the alignment of each machined laminate in the appropriate sequence to produce the device having the desired internal architecture and flow-through characteristics.
Bonding is the joining of the registered stack of formed laminates to produce a sound device that allows for fluid flow without leaking. Joining of the registered stack can be accomplished by diffusion bonding, reactive bonding, or diffusion soldering techniques. Diffusion bonding and reactive bonding methods are accomplished by vacuum hot-pressing or Hot Isostatic Processing (HIP).
Diffusion bonding is a process by which two nominally flat interfaces can be joined at an elevated temperature (typically about 50%-90% of the absolute melting point of the parent material) using an applied pressure for a time ranging from a few minutes to a few hours. Diffusion bonding produces a monolithic joint through the formation of bonds at the atomic level, as a result of closure of the mating surfaces due to the local plastic deformation at elevated temperature, which aids interdiffusion at the surface layers of the materials being joined. It is generally a two-stage process. In the first stage, asperities on each of the surfaces deform plastically as pressure is applied. These asperities arise from the grinding or polishing marks that have been produced in the surface finishing stage. The microplastic deformation proceeds until the localised effective stress at the contact area becomes less than the yield strength of the material at the bonding temperature. As the deformation of asperities proceeds, more metal-to-metal contact is established. Typically, at the end of the first stage, the bonded area can be less than 10% with a large volume of voids remaining between localized bonded regions. In the second stage of bonding, thermally activated mechanisms (creep and diffusion) lead to void shrinkage and this increases further the bonded areas. For conventional joining of substance layers via diffusion bonding, the processing parameters (pressure, temperature, and time) are set to optimum values based on the strength of the bond one wishes to create between the components. Diffusion bonding of a component with a microchannel made by the precision machining of foils can be complicated by the embedded internal channel or feature.
FIG. 1 is an exploded view of a registered stack of laminates, designated generally as numeral 10, before diffusion bonding. In this instance, the stack depicts a microlaminated two-fluid, interleaved, counterflow, microchannel array. A first set of alternating laminates 12, called fins define apertures 14. These apertures lie in registration with each other and with corresponding apertures 16 defined by a second set of intercalating laminates 18. This particular assembly facilitates fluid flow in opposite directions of two liquids whereby the two liquids never contact each other.
A heat exchanger of this general type, more particularly a multi-pass crossflow jet impingement heat exchanger is described in U.S. Pat. No. 5,016,707 awarded to Nguyen on May 21, 1991.
The diffusion bonding of a laminate stack relies on application of uniform uniaxial pressure to the laminate stack in a direction generally perpendicular to the planes in which the stacked laminates generally lie. FIG. 2 is a front schematic plan view of one such laminate stack, intended to produce a two-fluid interleaved, counter-flow microchannel array. Principal coordinate axes x, y, and z are also indicated for reference. The stacked laminates generally lie in the x-z plane, and uniaxial pressure P is applied in the direction of the y-axis. As illustrated in FIG. 2, the pressure P is transmitted through the stack via interfacial contact areas between adjacent laminae. Issues with the pressure transmission arise, however, when the internal geometry desired in the stack demands that gaps exists between certain adjacent laminae, such that some regions of individual laminae within the stack are unsupported by immediately adjacent laminae during the diffusion bonding. For example, the solid laminates 13, 15, and 17 shown at FIG. 2 positioned between gaps 19 and 21. Gaps 19 and 21 prevent the transmission of pressure P directly to the interfacial contact area 23, between solid laminates 13 and 15, and interfacial contact area 25, between solid laminates 15 and 17. Instead, the pressure felt at interfacial contact areas 23 and 25 is reliant on forces transmitted by solid laminates 13 and 17 over areas were solid laminates 13 and 17 are unsupported. This causes solid laminates 13 and 17 to experience bending moments, and reduces the pressure felt at interfacial contact areas 23 and 25. The pressure reduction can be significant for a situation such as that shown in FIG. 2, where the ultimate deflection of solid laminates 13 and 17 are constrained only by the elastic properties inherent in the laminae material. The situation represented in FIG. 2 is inevitable in the construction of cross-flow micro-channel heat exchangers. As a result, in the region where solid laminates 13, 15, and 17 are adjacent, the pressure felt at interfacial contact areas 23 and 25 is reduced and sufficient bonding between solid laminates 13, 15, and 17 may not occur. At a minimum, the strength of the bond formed at interfacial contact areas 23 and 25 is reduced due to increased porosity between laminates as a result of the locally reduced pressure. In the most egregious cases, the pressure reduction may be so severe that bonding in these regions does not occur at all. As an example, FIG. 6 illustrates a laminate stack similar to that shown at FIG. 2 and subjected to a uniaxial pressure for the purpose of diffusion bonding. As illustrated at FIG. 6, the uniaxial pressure applied has deformed the laminate stack to an extent where the interfacial contact areas between solid laminates 63 and 65 have experienced no diffusion bonding, and the interfacial contact area between solid laminates 65 and 67 are bonded inconsistently. A more complete explanation of this phenomenon is found in B. K. Paul, H. Hasan, J. S. Thomas, R. D. Wilson and D. Alman, Limits On aspect Ratios In Two-Fluid Micro-Scale Heat Exchangers, Transactions of NAMRI/SME, Gainesville, Fla., Vol. XXIX, 461-468 (2001) and incorporated herein by reference.
Realization of this low pressure problem has prompted fabricators to apply high pressures and temperatures. This sometimes distorts or collapses the spaces such as apertures and channels. The final result has been the production of devices with inferior flow properties.
Channel distortion is a function of the dimension of the width of the channel. The fin aspect ratio (channel width to fin lamina thickness) cannot exceed a certain value. The aspect ratio of concern is the aspect ratio of the fin that separates the channels. This ratio is defined as the distance that the fin is unsupported in the reactor design (the distance or length the fin is spanning the channel) divided by the thickness of the fin.
When producing micro-reactors via conventional hot-pressing, there is a limiting aspect ratio for a given uniaxial pressure which if exceeded will distort the fins and produce a structure which leaks or has poor fluid flow properties. Therefore, there will be some optimum balance between fin aspect ratio and the uniaxial pressure at which all lamina will uniformly and effectively bond. This is described in B. K. Paul, et al. supra. Paul describes the fundamental limit on the size of the internal feature which can be produced via diffusion bonding methods using a given uniaxial pressure, e.g., through hot-pressing. The optimum balance necessary either limits the size of the internal feature which can be produced through diffusion bonding of precision machined foils, or limits the uniaxial pressure which may be applied to produce adequate and uniform diffusion bonding, both of which may limit the performance characteristics of the micro-channel reactor.
The distortion can be avoided by the use of sacrificial cores and internal gaskets to provide support during the vacuum hot-pressing process. This method is described in U.S. Pat. No. 5,317,805 awarded to Hoopman et al., on Jun. 7, 1994, and U.S. Pat. No. 5,269,058 awarded to Wiggs et al., on Dec. 14, 1993.
U.S. Pat. No. 6,464,129 awarded to Stueber, et al. on Oct. 15, 2002 discloses a method for joining superalloy substrates using diffusion bonding. The method entails depositing an activator directly on the surface of the joint to be bonded without the use of a brazing alloy.
U.S. Pat. No. 6,129,261 awarded to Sanders on Oct. 10, 2000 discloses a method for compression diffusion bonding. The method entails the use of a Corrosion Resistant Steel (ORES) template to apply increased pressure in the areas designated for diffusion bonds.
U.S. Pat. No. 5,284,288 awarded to Woodward on Feb. 8, 1994 discloses a method for the manufacturing of an article by diffusion bonding. The method includes the use of welding to weld edges of metal sheets together, and a stop off material on surfaces to prevent diffusion bonding at predetermined positions. The binder in the stop off material is subsequently removed.
U.S. patent application Ser. No. 10/576,963 by Paul, et al, filed Oct. 25, 2004, discusses a pre-bonding step wherein sub-sections of laminae prone to warpage and channel collapse are tack-bonded prior to application of full uniaxial diffusion bonding pressure to the entire stack. Paul indicates the pre-bonding step may be a diffusion bonding at low pressure, resistance spot welding, or other techniques. The purpose of the pre-bonding technique is to ensure that all areas of adjacent laminae in the sub-section experience intimate contact during the subsequent application of uniaxial pressure for final bonding of the entire stack. This approach may alleviate issues of unconstrained bending leading to incomplete bonding as a result of inadequate contact, however in the application of subsequent uniaxial pressure, non-uniform pressure gradients across unsupported internal laminate spans still occur and inadequate and/or non-uniform bonding may still result in some regions.
A need exists in the art for a graduated diffusion bonding process which provides for the application of uniform pressure to all regions of a stacked laminae during final bonding while maintaining an intimate contact at interfacial contact areas, such that all regions in the stacked laminae experience substantially the same temperature and pressure conditions during final bonding, thereby producing substantially uniform bond strength at interfacial contact areas throughout the laminae stack. Such a process should prevent collapse of internal, embedded features during conversion of a laminate stack into a monolith structure. The process should obviate the need for sacrificial cores, internal gaskets, templates, brazing alloys, or binders. Also, the process should not limit the size of the embedded internal features of the article being manufactured.
Thus, it is an object of the present disclosure to provide a method for producing components with internal architectures which overcomes many advantages of the prior art.
It is another object of the present disclosure to provide a process for producing monoliths having internal structures which maintain their configuration during fabrication of the monoliths. A feature of the invention is the application of a graduated diffusion bonding process. An advantage of the invention is that the process maximizes monolith density and minimizes leaking of fluid from within the various internal structures.
It is another object of the present disclosure to provide a process for formation of microstructures, such as microchannels which do not experience any significant distortion during processing. A feature of the invention is a graduated application of diffusion bonding to layers of substance such that the layers are initially weakly bonded together before further treatment. An advantage of this invention is that fluid flow characteristics through channels is not degraded by the distortion of the channels from processing. An additional advantage of the invention is that pores and unintentional voids present in the laminates are greatly diminished.
It is another object of the present disclosure to provide a process which can create a properly sealed solid article via diffusion bonding. A feature of the invention is that during initial steps of the process, a combination of lower pressure (as low as 100 pounds per square inch (psi)), lower temperature (as low as Th=0.4; Th=homologous temperature: process Kelvin temperature divided by Kelvin melting temperature of the material), and shorter time periods (as short as 0.25 hour (hr)) than in conventional one-step hot-pressing diffusion bonding methods is used. An advantage of this feature is that it allows for prebonding of the laminates in regions adjacent to the embedded features while also providing a means for preventing collapse or distortion of those regions.
It is another object of the present disclosure to provide a process for diffusion bonding of laminae arranged in registration, during which the embedded features such as channels do not collapse. A feature of the invention is the use of very high pressure during processing steps (as high as 30 ksi (30,000 psi)) which keeps channels intact due to their being pressurized by the high pressure itself. Another feature of the present invention is that there is no limiting aspect ratio requirements of fin laminae that separates channel laminae. An advantage of this feature is that supplementary materials such as sacrificial cores, internal gaskets, and templates are not used, thus lowering costs.
It is another object of the present disclosure to provide a process which maximizes processing efficiency of diffusion bonding and structural integrity of the device being made. A feature of the invention is that a uniaxial low pressure step, then a isostatic high pressure step can be performed entirely within an HIP apparatus. An advantage of this feature is the elimination of steps that are otherwise normally required for cooling of the pre-bonded laminae stack and moving the stack between different and separate apparati for the two steps. This leads to greater efficiency of the two-step process and lower costs.